The SPR (Special purpose receiver) is a cmmg upper with a threaded medium/heavy 16.5" WASP barrel (18" with A2 flash hider). If you have a high end piston upper I will include in the trade a complete xm15-E2S lower with all new parts kit installed. Less than 100 rounds through lower. 500 rounds through upper. The piston upper im looking for must be in excellent condition with a pencil or lightweight /M4 contour barrel.